Fix usr.sbin/rpcbind ATF tests on 32-bit platforms

usr.sbin/rpcbind/tests/addrmerge_test.c
Fix some sizeof calculations that work only by luck on 64-bit
platforms.

PR: 210314
Reviewed by: ngie
Approved by: re (kib)
Reported by: Mark Millard
MFC after: 4 weeks
Sponsored by: Spectra Logic Corp
Differential Revision: https://reviews.freebsd.org/D6855